This invention relates to the modification of supercooled clouds. The principle of the current large-scale weather modification method, including the technique of rain augmentation seeding, involves artificial generation of ice crystals in supercooled clouds and fogs with a resultant spontaneous phase change. There are two major effects to be utilized out of the phase change: (1) one to make use of the heat of the phase change to induce a new cloud dynamics or motion, and (2) the other to use the breakdown of the colloidal stability of the cloud through ice crystal growth and fall with simultaneous cloud droplet evaporation. In the latter, it becomes artificial snow when the ice crystal reach the ground without melting and artificial rain when the crystals melt after falling through the 0.degree. C. isotherm.
Clouds evaporate, at least partially, even in the stage of their formation by entraining or mixing with the surrounding dry air, and their lifetime is limited. Precipitation, including rain, snow, graupel and hail, must form within the limited lifetime of the cloud.
Precipitation forms by two major mechanisms in the cloud: "warm rain" forms by collision-coalescence of cloud droplets, and the other first nucleates ice crystals in a supercooled cloud, and the crystals grow by evaporating the cloud droplets with vapor diffusion or by allowing collisions with the droplets to freeze on the crystal and fall. In the former, growth of ice crystals represents the mechanisms of snowfall; and in the latter, the growth of graupel and hail. When the formed ice crystals fall through the 0.degree.C. isotherm and melt, they become "cold rain." It is known that these mechanisms of precipitation formation, e.g. ice crystal formation in large numbers, do not take place efficiently in natural clouds. Thus, there exists a space for artificial control of cloud processes by supplementing the ice crystals.
The efficiency of the precipitation process is influenced by the updraft in the cloud induced by the generated latent heat of condensation during the formation. When the updraft is too strong, the natural ice crystal process, particularly the snow crystal growth, cannot fully proceed due to lack of time. The cloud under this condition rapidly goes up without being influenced by the growing ice crystals, reaches the upper portion of a low temperature, about -40.degree. C., and freezes to become small ice crystals. The small crystals consisting of the anvil of the dynamic cloud form by this mechanism, tend to be blown out of the cloud and evaporate in very large quantities in the atmosphere without being involved in precipitation formation processes.
In the process of natural precipitation formation, the ice nucleation mechanisms are often found to be deficient as mentioned above. Under this condition, by artificially introducing ice crystals, it is possible to modify the process. There exist two methods of artificially generating the ice crystals, i.e., by homogeneous and heterogeneous ice nucleations. The former works when a portion of the cloud is chilled below -40.degree. C. by a strong coolant. This method generates ice crystals in large numbers without any help from existing particles or other substances. The mechanism of the ice formation is believed to be largely due to condensation of water vapor in the form of small droplets followed by their freezing due to the strong cooling. The efficiency of ice crystal generation in numbers per gram of coolant increases as the temperature drops towards -60.degree. C., about where the increase naturally ceases. Dry ice is an example of the coolants, and when placed in the air, the temperature reaches -100.degree. C. due to evaporative cooling and generates a large number of ice crystals.
The second category of ice nucleants is called ice (forming) nuclei, and ice forms on the individual particle of the nucleus substance. Silver iodide (AgI) and metaldehyde are examples of artificial ice nucleus substances. AgI, the most widely used nucleus substance for weather modification, is expensive and toxic to small or baby fish, algae and bacteria. The mechanism of ice nucleation is complex, and the ice nucleating ability sharply depends on cloud temperature, reducing it by a factor of 10.sup.3 for a temperature rise from -10 to -5.degree. C. The ability is lost at temperatures above about -4.degree. C. Metaldehyde is inexpensive and effective up to a higher temperature, shows no environmental toxicity, and the number of ice crystals generated per gram compound is slightly lower than that of AgI at low temperatures, but the number still decreases as the temperature rises towards 0.degree. C.
AgI is a solid under room temperature, like metaldehyde, and the particles can be released from the ground without melting or evaporating. Under sunshine, the particles are known to slowly lose their ice nucleating ability. Most importantly, the AgI smoke by itself cannot effectively diffuse into the large space of a cloud. A cumulus cloud grows due to an updraft at the base. If and when the ground-seeded AgI smoke reaches there or the smoke is directly laid by an airborne seeding, it will be taken into the cloud. However, the air temperature is higher at the low level in cloud where ice nucleation by AgI smoke particles does not function well, and the smoke will be carried upwards by the organized updraft without substantial ice nucleation and spreading by turbulent diffusion to the top portion, where now an effective ice nucleation proceeds in a relatively small cloud volume, but the volume limitation severely restricts the growth process resulting in small crystals. The small crystals formed do not fall well and dynamically stabilize at the cloud top position with the help of heat generated by the phase change. This stabilized condition averts further changes, and most of the crystals formed eventually evaporate without causing a significant amount of precipitation.
In the natural supercooled clouds, depending on the type, age and the place of formation, ice crystals normally start appearing at around -10.degree. C., and they become substantial when the temperature reaches about -20.degree. C. Artificial introduction of ice crystals in such a cloud to supplement the natural deficiency is thus useful at temperatures above about -10.degree. C.
A method to drop dry ice pellets from above a supercooled cloud has also been used occasionally. A falling dry ice pellet generates ice crystals by about 10.sup.13 /g almost independently of the cloud temperature. This has been considered as an advantage because the seeding aircraft does not have to fly into the cloud. The generated ice crystal thermals from falling dry ice pellets are, however, oriented vertically along the routes of pellet fall. These vertical ice crystal thermals are warmer than the surrounding supercooled cloud volume due to the heat generated by the phase change from the supercooled cloud droplets to ice crystals. The warming is as much as 0.7.degree. C. when the liquid water content is 1 gm.sup.-3 in the standard atmosphere. This warming is equivalent to the temperature difference between a typical growing cumulus cloud and the surrounding air. The ice crystal thermal in the shape of a vertical ribbon at the beginning integrates the buoyancy force in the vertical direction, and just like cigarette smoke, drives itself upward. The thermal thus moves up rapidly, and the fast motion reduces the time for the thermal to diffuse into and mix with the surrounding supercooled cloud volume as well as that of ice crystal growth under the necessary condition of being mixed with the supercooled cloud volume. The resultant thermal of small crystals spreads at the top of the cloud without fall, or sometimes to protrude a "finger" out of the cloud, but in a small volume. Due to buoyancy, the thermal stabilizes there without effectively developing any precipitation.
The above description of the problems associated with the contemporary methods of cloud seeding can be solved by a new method claimed by the present invention which selects effective fundamental processes and incorporates their feedback to optimize the development of the precipitation elements and their subsequent spreading into the entire cloud space, thereby maximizing the dynamic effect as well, and radically advances cloud seeding technology. The superior seeding effect claimed in the present invention has been confirmed by seeding tests.